CMake: creating object and executable inplace

Question

I have the following setup:

data/
  Makefile
src/
  main.cpp
CMakeLists.txt

The data/Makefile creates a file data/file via the all target.

The src/main.cpp needs to read data/file, e.g.

// src/main.cpp
...
      auto is = std::ifstream{"data/file"};
...

How can I specify that the executable target for src/main.cpp should depend on the output of data/Makefile?

Answer 1

The CMake documentation for add_custom_target states

Adds a target with the given name that executes the given commands. The target has no output file and is always considered out of date even if the commands try to create a file with the name of the target. Use the add_custom_command() command to generate a file with dependencies. By default nothing depends on the custom target. Use the add_dependencies() command to add dependencies to or from other targets.

Using that hint I found the following way to work:

The top-level CMakeLists.txt

# CMakeLists.txt
add_subdirectory(data)
add_executable(my_exec src/main.cpp)
add_dependencies(my_exec data_files)

The CMakeLists.txt in data/

# data/CMakeLists.txt
add_custom_command(OUTPUT ${CMAKE_CURRENT_SOURCE_DIR}/file
                   COMMAND make all
                   DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/Makefile 
                   WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
                   )
add_custom_target(data_files
                  DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/file)